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THE CHIMNEY OR STACK EFFECT EXPLAINED

The natural phenomena of the density difference between a hot and cold air column that creates a natural

flow through a chimney is called the Chimney effect. Learn more about this in this article.

You can see the tall flue gas stacks in all the power plants. The function of the stack is to disperse the hot

gases, emissions and particulates that leave the boiler to a great height. At these heights the pollutants

disperse in a very large area so that ground level concentrations are within permissible levels not harmful for

humans or vegetation.

Chimneys where in use from the times of the Roman Empire. Chimneys and fireplaces are a common

household item in countries with a cold climate. It does the dual function of removing the hot gases out of

the house at the same time bringing in fresh air to the fireplace for combustion.

Flue gas stacks higher than 250 meters are common nowadays for larger power plants. The tallest stack

currently is 420 meters in Kazakhstan. Many factors like terrain, dispersion pattern, plume heights, adjacent

tall structures, and population density determine the height of the stack.

There is a natural phenomena associated with the chimney or the flue gas stack. This is the natural flow of

air up the chimney. This is called the ‘chimney or the stack effect’. This effect is found not only in chimneys

but also in tall buildings.

What is the Chimney (or Stock) Effect?

The gas temperature inside the flue gas stack is around 140 ° C. The outside ambient air temperature is

around say 30° C. Consider this as two air columns connected at the bottom. The high density and heavier

cold air will be always pushing the low density and lighter hot gases up. This causes the natural flow of

gases up the flue gas stack. This pressure difference that pushes the hot gas up the flue gas stack or the

chimney is the 'chimney or stack effect'.

You can feel the effect if you stand near the doors or openings at the bottom of a stack or at open door of an

elevator shaft. Depending on the height it can be gentle draught or heavy suction. This is the chimney or

stack effect.

In numerical terms this can be represented as

Chimney effect = 353 x Chimney Height x [1/ Stack gas temperature – 1/ Ambient Temperature]

26 Comments

Article by johnzactruba (5,472 pts )

Edited & published by Haresh Khemani (30,568 pts ) on Feb 3, 2010

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of 4What is the Chimney Effect or Stack Effect?

14.01.2011http://www.brighthub.com/engineering/mechanical/articles/29769.aspx

Where

Chimney effect is in mm of water column.

Chimney height is in mteres.

Temperatures are in ° Kelvin.

For a thermal power plant with a stack height of 250 meters the effect could be around 77 mm of water

column. In thermal power plants the stack effect aids the Induced draft fans in removing the hot flue gases

from the furnace and dispersing them at the top of the stack.

In tall buildings this effect could create problems for the airconditioning system. In deserts where the outside

temperatures are higher than the cool interior of the buildings the effect will be in the reverse.

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alok bhattacharya Nov 27, 2010 10:15 AM AFBC boiler

I am not getting maximum output from the plant,i.e. 7MW instead of 10 MW. Can it be because of erratic draft pattern from boiler to I/D fan? I am planning to take shutdown of plant and check complete generating section of boiler. I guess there is external air ingress into the system as such draft is not stable. Even after altering fuel feed (Coal +rice husk), I am not getting max. pressure which may causing low generation. I hope, I could explain my problem and your guidance will help me to fix the problem. You can understand the financial loss because of 3MW loss continuously. Regards, Alok Bhattacharya 27th,Nov,2010.

johnzactruba Nov 25, 2010 5:26 PM reply Aman

Stack effect depends depends on the temperature of the gas. If load changes and if there is gas temperature change it affects the stack effect. AS the load on the boiler increases the volume of gas flow increases, the friction loss in the stack also increases. The nett efefct (Stack effect - loss ) will be less at higher loads.

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Aman Nov 25, 2010 10:00 AM relation b/w draft & load on boiler

Sir, If load on boiler is increased, will the chimney draft affected.

Hemant Sharma Nov 23, 2010 1:44 AM stack height for automobile,stack thickness

what is permissible limit of outlet of variou gas,what is the permissible limit of height of stack,what is the permissible limit of thickness ,enclosure of chimeny in auto industry

johnzactruba Jun 21, 2010 6:00 PM reply GL

normally the the chimney is sized such that the stack effect and the chimney losses are equal., so that at exit of ID fan the pressure is almost zero. But if the stack is not sized properly the ID Fan will have to do more work to push the gases through the stack. In these marginal aces the effect of ambient temperature will be experienced. Leakage before of the ID fan is the single biggest cause for the ID fan to go to maximum lod condition.

G L Jun 21, 2010 4:03 PM Stack Effect

Would the stack effect affect the ID fan performance? During summer time (warmer temperatures) ID fans will tend to saturate (inlet dampers 100% open) while trying to maintain furnace draft (-0.5 in H2O). We've had to lower the load to maintain the furnace in a negative draft. We've looked for air in-leakge upstream of the ID fans and have fixed a couple of cracks. Would cracks/holes downstream of the ID fans affect the stack effect or ID fan performance? Thanks!

o.p.tandon Apr 18, 2010 5:19 AM permissible value

i would like to know permissible limit value of spm sox , nox in thermal power plant. Thanks sir

alok bhattacharya Apr 1, 2010 4:03 AM ESP

As I understand, before putting flue gases into atmosphere, it is been passed through electrostatic precipitator to catch ash/ unburnt coal particle. In fact it has become mandatory to put ESP before I/D fan by pollution board. Your comment please. Moreover I would like to know the permissible limit of NOX, SOX, Co and SPM at present senerio.

jimmy yeo Mar 31, 2010 3:06 PM renewable energy

have it ever been considered that stack effect be used : 1.as a free and renewable energy? the draft can certainly turn a fanblade inside the chimney to generat3 electricity? 2. for pollution control. in large cities by attaching several chimneys to every buildings sucking the fumes generated by motor vehicles to above the top of the buildings? 3. evirnmental control, to counter greenhouse effect, by drawing minute water particles generated by hypersonic vibrations peizo, into the atmosphere to create a blanket to deflect sunlight?

rajesh Mar 10, 2010 3:13 AM SIGNIFICANCE OF THE NUMBERS

We happen to assign a value to the effect by calculating the chimney effect. Kindly brief with regard to the industrial chimney and high rise buildings the significance of these numbers. For instance the tallest building BURJ KHALIFA- height 828m, ambient o/s temp 50 deg C in summers and int temp say 23 deg C

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